The invention relates generally to fiber optical chemical sensors (FOCS) and other chemical sensors and more particularly to the elimination of sensor measurement errors caused by instrumental fluctuations, variations in optical properties, changes in temperature and slow loss of sensing material due to leaching or bleaching.
A fiber optic transmits light by total internal reflection at the core/clad interface when the refractive index of the clad is less than the index of the core. Optical fibers have been used in a wide variety of sensors, known as "optrodes" or "fiber optic chemical sensors" (FOCS), which can detect various chemical species or physical parameters. Illustrative U.S. Pat. Nos. include 4,582,809 to Block, et al.; 4,321,057 to Buckles; 4,399,099 to Buckles; 4,682,895 to Costello; 4,600,310 to Cramp, et al.; 4,509,370 to Hirschfeld; 4,542,987 to Hirschfeld; 4,447,546 to Hirschfeld, et al.; 4,634,856 to Kirkham; 4,846,548 to Klainer; 4,892,383 to Klainer, et al.; 4,824,206 to Klainer, et al.; 4,913,519 to Klainer, et al.; 4,272,485 to Lubbers; 4,269,516 to Lubbers; 4,476,870 to Peterson, et al.; 4,200,110 to Petersen, et al. U.S. patent applications Ser. No. 503,463 now U.S. Pat. No. 5,059,790, to Klainer, et al.; Ser. No. 501,144, now U.S. Pat. No. 5,026,139, to Klainer, et al.; Ser. No. 501,146 to Klainer et al.; Ser. No. 004,339, abandoned, to Walt, et al. also illustrate fiber optic chemical sensors.
In addition to fiber optic type chemical sensors in which the fiber optic is used to transmit excitation and detector signals to and from the active sensor, other chemical sensors have been developed in which the fiber optic has been illuminated. In particular, U.S. patent application Ser. No. 544,681 to Klainer, et al. describes a class of reservoir type chemical sensors in which the source and detector are mounted within the cell body.
Typically an analyte specific reaction chemistry (e.g., an organic dye, transition metals, and their complexes or charge transfer complexes) is immobilized on the FOCS or placed in the chemical sensor cell body. The absorption or emission intensity is modulated as these indicator materials interact with the specific analytes, thus producing a detector signal which provides a measure of the analyte of interest. For example, Ru(bpy).sub.3.sup.2+ (Rutheniumtrisbipyridyl) which fluoresces with a peak at 610 nm, when excited at 440 nm, exhibits oxygen sensitive luminescence. In the presence of oxygen, Ru(bpy).sub.3.sup.2+ luminescence is quenched and this quenching depends on the concentration of oxygen.
However, FOCS and other chemical sensors often exhibit poor performance because of degradation or other variations of the sensor. Sensor problems generally result from temperature variations, differences between coatings, light (illuminator) intensity changes and flicker, loss of indicator material and fouling of the sensor. For fluorescent sensors a particular problem is bleaching of the dye. Any of these variations, or others of a similar nature, cause changes in the sensor output which do not depend on the analyte, and thus produce an inaccurate measurement. For example, with reference to the oxygen sensor described above, if the Ru(bpy).sub.3.sup.2+ undergoes degradation, or even if some Ru(bpy).sub.3.sup.2+ leaks out of the sensor, then the emission intensity of Ru(bpy).sub.3.sup.2+ will decrease, producing an erroneous oxygen reading. Thus it would be highly advantageous to have a FOCS or other chemical sensor in which the output is solely a function of the analyte or parameter of interest, and in which other variations in the sensor are compensated for so that they do not affect the sensor output. In particular, the objective is to use the FOCS or other sensor itself to provide the input for the necessary corrections.